The technical field of this invention is vehicle brake control.
Many vehicle brake systems include supplemental apply capability, wherein one or more wheel brakes may be applied by the brake control without initiation by the vehicle operator. An example of such a system includes a brake control with a vehicle stability enhancement (VSE) system, in which a selected wheel brake of the system may be applied to counter an undesired vehicle yawing motion in closed loop control of vehicle yaw rate. Such a system may include an electric motor driven pump for providing braking fluid pressure to the selected wheel and a normally open isolation valve between the master cylinder and the individual brake apply valves that is closed during a supplemental apply to permit braking pressure at the brake apply valves, and thus at the selected brake, to exceed master cylinder pressure as required for the supplemental apply. Such a system further has a normally closed prime valve that is open during the supplemental apply to permit fluid to be supplied as required to the pump inlet from the master cylinder. A blow-off capability is provided at a predetermined high pressure, preferably in the isolation valve but alternatively or additionally in a separate blow-off valve, the blow-off action of the valve being provided when fluid pressure overcomes an electromagnetic force created by an electric current in a coil applied to close the movable valve element and the electric current preferably being controlled by the system controller to establish the predetermined blow-off pressure. The blow-off capability permits excess fluid to escape back to the master cylinder when the pressure of the fluid provided by the pump to the apply valves exceeds the predetermined blow-off pressure. The brake apply and release valves for the regulated wheel brake are operated to regulate the pressure of pump supplied fluid to the regulated wheel brake. Two parallel fluid circulation paths are established through the pump, one through the brake apply and release valves of the regulated wheel brake and another through the isolation or blow-off valve and master cylinder. Each of these circulation paths includes a fluid reservoir open to the pump inlet: an accumulator in the one path through the apply and release valves and the master cylinder itself in the other path supplementing the capacity of the accumulator. Blow-off action, with its undesirable noise, can be common during a supplemental apply, since the pump normally maintains pressure in the brake lines to the apply valves at a high pressure just under the blow-off pressure and a significant amount of fluid, entering the pump from the master cylinder through the opened prime valve, must be returned to the master cylinder.
If the vehicle operator manually applies the brakes during a supplemental brake apply, pressure in the master cylinder and the brake lines between the master cylinder and the isolation valve is raised. The blow-off action of the isolation or blow-off valve tends to produce audible noise as the moving valve element slams back against the seat after each pulsed release of fluid; and this noise is more easily transmitted to the passenger compartment when the pressure in the master cylinder is high. The noise can be objectionable to vehicle occupants and can occur as long as the brake pedal is depressed during a supplemental brake apply. A reduction in this noise would provide more transparent operation of the supplemental brake apply and greater operator satisfaction with the vehicle in which the brake system is installed.
In this invention, the noise produced during a supplemental brake apply is reduced, and possibly eliminated, by reducing the necessity for blow-off of excess brake fluid back to the master cylinder. This is accomplished with no change to the basic fluid conduction apparatus of the brake system (except for the addition of a pressure sensor to measure master cylinder pressure, if it is not already present), by changing the way in which the valves of the apparatus are controlled. Although the prime valve is opened at the beginning of a supplemental apply, it is closed after a predetermined quantity of brake fluid is provided to an accumulator connected to the pump inlet, for example by keeping the prime valve open only for a predetermined time period. This quantity of fluid is sufficient to ensure fill of the brake lines and at least partial fill of the accumulator but is preferably sufficiently small so that all excess fluid likely to be required during a supplemental brake apply can be retained in the accumulator. When the quantity of fluid has been provided, the prime valve is closed and preferably remains closed for the duration of the supplemental apply.
The method of this invention can operate with a smaller quantity of excess fluid because of the way in which the apply and release valves are operated. With the prime valve closed, a fluid pressure in the master cylinder is sensed, a first wheel brake apply valve is maintained in an open condition and a first wheel brake release valve is maintained in a closed condition. The first wheel brake apply and release valves are those connecting the pump pressure output to the first (regulated) wheel brake of a pair of wheel brakes such as one half of a diagonal split braking system; but, unlike the prior art, these first apply and release valves are not used for regulation of the first wheel brake. Instead, a second wheel brake apply valve, connecting the pump pressure output to the second (non-regulated) wheel brake of the pair and connected on the pump side to the first wheel brake apply valve, is controlled to regulate fluid pressure from the fluid source so as to provide a predetermined regulated fluid pressure to the second wheel brake apply valve and, by the back pressure produced, through the open first wheel brake apply valve to the first wheel brake. A second wheel brake release valve is controlled to maintain the second wheel brake at the sensed fluid pressure in the master cylinder, which pressure will be lower than the predetermined regulated fluid pressure as long as the vehicle operator does not activate the brakes manually to increase the master cylinder pressure above the predetermined regulated pressure.
In this method, there is no brake fluid circulation through the first brake apply valve of the regulated wheel brake to the accumulator or pump, although the first brake apply valve is fully open, since such circulation is blocked by the closed first brake release valve. And, after the prime valve is closed, there is rarely, if ever, fluid flow through the parallel fluid circulation path of the prior art via the blow-off valve. Essentially, a single fluid circulation path for the pump is established through the second brake apply and release valves of the second (non-regulated) wheel brake and the accumulator. This method provides regulation of the first wheel brake with a more even fluid circulation and thus less need for additional fluid from the accumulator and/or master cylinder. The quantity of excess fluid required is sufficiently small to be stored in the accumulator, and this permits the closure of the prime valve after it has been provided. In addition, brake fluid in the brake lines between the fluid pump and the first and second brake apply valves is maintained at the regulated pressure, which is generally significantly lower, and thus further below the blow-off pressure, than the pressure in these lines using the prior art method. There is thus essentially no need in a supplemental brake apply for blow-off action to release excess fluid to the master cylinder, and objectionable noise is greatly reduced. As an additional benefit, the maintenance of brake pressure at the non-regulated wheel in response to a master cylinder pressure sensor results in at least some braking by the second wheel brake when the vehicle operator manually activates the brakes via the brake pedal and master cylinder during a supplemental brake apply, whereas in the prior art method the second wheel brake apply valve is maintained closed during a supplemental brake apply and the second wheel brake is thus not used.